[Concurrency] Allow sendability mismatches while overriding `@preconcurrency` properties in Swift 5 mode
Downgrade a mismatch on `any Sendable` -> `Any` to a warning until
Swift 6 to enable class authors to introduce concurrency annotations
to overridable properties.
Resolves: rdar://122193606
(cherry picked from commit cdf28bc48247901995fc62689c93b94164a53aa9)